1. Field of the Invention
This invention relates to an optical pickup apparatus for applying a light beam from a light source such as a semiconductor laser to an optical memory medium and effecting one or both of recording and reproduction of information.
2. Description of the Prior Art
In recent years, studies of optical information processing apparatuses utilizing a laser or the like to optically record or reproduce information have been actively carried out. For example, apparatuses using a medium exclusively for use for reproduction represented by a video disc or the like have already been manufactured, and further, apparatuses using a DRAW (direct read after write) medium capable of additional recording or an erasable opto-magnetic recording medium are regarded as being promising. The photo-magnetic recording medium utilizes the localized temperature rise of a magnetic thin film caused by the spot application of a laser light thereto, whereby information is magnetically recorded and is reproduced by the magneto-optic effect (especially the Kerr effect). The Kerr effect refers to the phenomenon that the plane of polarization rotates when light is reflected by a magnetic material.
In the optical information processing apparatus as described above, a light beam is applied to a medium through an optical pickup apparatus. FIG. 1 of the accompanying drawings is a schematic view illustrating the construction of such a conventional optical pickup apparatus. In FIG. 1, reference numeral 1 designates a semiconductor laser, reference numeral 2 denotes a collimator lens, reference numeral 3 designates a polarizer, reference numeral 4 denotes a polarizing beam splitter, reference numeral 5 designates an objective lens, reference numeral 6 denotes an opto-magnetic disc, reference numeral 7 designates an analyser, reference numeral 8 denotes a condensing lens, and reference numeral 9 designates a detector surface.
A light beam emitted from the semiconductor laser 1 is collimated by the collimator lens 2 and is made into linearly polarized light by the polarizer 3. In FIG. 1, the plane of P-polarization is parallel to the plane of the drawing sheet, the plane of S-polarization is perpendicular to the plane of the drawing sheet, and the plane of polarization of the semiconductor laser 1 is parallel (the direction of P-polarization) to the plane of the drawing sheet. If the amplitude transmittance of the P-polarized light component through the polarizing beam splitter 4 is t.sub.P and the amplitude reflectance of the S-polarized light component is r.sub.S, it is known that in the optical system of FIG. 1, a signal of good S/N ratio is obtained if r.sub.S is chosen to have a great value and t.sub.P is chosen to have a suitable value (for example, t.sub.P.sup.2 =70%, r.sub.S 2=98%). (See Japanese Laid-Open Patent Application No. 200958/1982.) The parallel light beam transmitted through the polarizing beam splitter 4 is imaged as a minute spot on the opto-magnetic disc 6 by the objective lens 5.
The reflected light from the opto-magnetic disc 6 is subjected to the rotation of the plane of polarization (Kerr rotation) in opposite directions in accordance with the direction of magnetization of the area to which the spot is applied, due to the Kerr effect, and is again made into a parallel light beam by the objective lens 5. When that light beam is reflected by the polarizing beam splitter 4, the angle of polarization of the light beam (i.e., the angle through which the plane of polarization is rotated) is apparently increased by the ratio between r.sub.P.sup.2 (=1-t.sub.P 2) and r.sub.S 2. Thereafter, the light beam is separated into a predetermined polarized light component by the analyser 7 and passes through the condensing lens 8 to the detector surface 9 as a signal light. Thus, detection of RF signal, focus control, tracking control, etc. are effected by the use of the conventional method. Also, recording and erasing of the information are likewise effected by this system.
Such a conventional apparatus has the following disadvantages:
(1) Stray light created by the separation surface of the beam splitter mixes with the signal light and provides noise; and PA1 (2) A circular spot is not obtained on the surface of the recording medium.
The disadvantage mentioned under item (1) above is attributable to the fact that the conventional beam splitter has a cubic shape. For example, in FIG. 1, part of the incident light beam from the semiconductor laser 1 is reflected by the surface A of the polarizing beam splitter 4, is reflected also by the surface B and returns to the surface A. With regard to the surface A, to improve the S/N ratio, as previously described, the transmittance t.sub.P and the amplitude reflectance r.sub.P of the P-polarized light component are chosen to satisfy t.sub.P 2=70% and r.sub.P 2=30%, respectively, and therefore, most of the light beam from the surface B is transmitted through the surface A and mixes with the signal light as a stray light. Since the modulated component of the signal light is weak, the stray light imparts a considerable influence even if the reflectance on the surface B is of the order of 1%.
The disadvantage mentioned under item (2) above is attributable to the fact that the light distribution characteristic of the high-output semiconductor laser is, for example, of the order of 30.degree. in a direction perpendicular to the joined surface (i.e., the beam splitting surface itself; A in FIG. 1) and of the order of 10.degree. in a direction parallel to the joined surface. Therefore, the shape of the beam is elliptical, as shown in FIG. 2 of the accompanying drawings, and, when the beam has been condensed as a minute spot on the recording medium, a circular spot has not been obtained, thus resulting in reduced recording density.